1. Field of the Invention
The present invention generally relates to an audio signal processing apparatus which is applied to digital audio communications systems in the mobile communications field of, e.g., portable phones and the like and, more particularly, to a noise suppression function or echo suppression function in audio coding.
2. Description of the Related Art
In general, in the mobile communications field of, e.g., portable phones and the like, a digital audio communications system is applied. The digital audio communications system adopts audio coding (compression coding) to transmit compressed audio data.
In the mobile communications field, a low-bit rate coding method called CELP (Code Excited Linear Prediction) is known as a typical audio coding method. Upon audio coding using such method, not only an audio signal but also an audio signal including noise components called high-frequency ambient noise is often encoded.
As is known, when an audio signal containing noise and echo components is encoded, encoded audio data with poor quality is generated. For this reason, an audio coding circuit adopts a noise suppression circuit called a noise canceller so as to input only an audio signal from which noise components are suppressed. Also, an echo suppression circuit such as an echo canceller, voice switch, or the like is adopted to input an audio signal from which echo components are suppressed.
The noise canceller determines a state wherein no audio signal is input, i.e., only an ambient noise signal is input. The noise canceller analyzes the feature of the ambient noise signal in that state. Then, the noise canceller suppresses noise components using the feature during a period in which an audio signal and noise components mix.
The echo canceller determines a state wherein an audio signal reaches the receiving side but no audio signal is output from the sending side, i.e., a single-talk state of the receiving side. The echo canceller learns the returned acoustic characteristics from the receiving side to the sending side in that state. Then, the noise canceller suppresses echo components that mix in a signal on the sending side using the learned acoustic characteristics. The voice switch compares the signal powers of the receiving and sending sides, and suppresses echo components by inputting a loss to the lower power side.
An audio coding scheme used in current portable phones is limited to the frequency band where an audio signal is mainly present. In recent years, a wideband coding scheme that implements audio coding in a frequency band wider than the audio signal frequency band is undergoing standardization. Such wideband coding scheme adopts CELP, and requires the noise canceller and echo canceller or voice switch.
In an audio signal processor which uses a noise canceller and adopts a wideband coding scheme, a digital audio signal routed via the noise canceller is divided into high-frequency audio signal components which have less power as an audio signal and are not important in terms of information, and other low-frequency audio signal components. High-frequency audio signal components are not necessary in a given coding mode, and a method of removing such components from encoded audio data is known. As the coding mode, for example, AMR-WB (Adaptive Multi-Rate Wideband) codec specified by the 3GPP (3rd Generation Partnership Project) standard is available.
In fact, in the coding mode that outputs encoded audio data of only low-frequency audio signal components (e.g., when the transmission rate is other than 23.85 kbps in AMR-WB), the noise canceller need not execute a noise suppression process for digital audio signal components of a full frequency band output from an A/D converter 11, and need only execute a noise suppression process for low-frequency audio signal components.
In general, the noise canceller comprises a digital signal processor (DSP). Therefore, when the noise canceller executes digital audio signal components of the full frequency band, an excessive data processing volume and memory size are required for the DSP upon implementing the noise canceller function.
The same applies to the echo canceller, and the audio signal processing efficiency are desirably improved by reducing the data processing volume and memory size required to implement an echo suppression function.
Note that a method of reducing the calculation volume and necessary memory size has been proposed, in which echo cancellation of only low-frequency audio signal components without that of high-frequency audio signal components is executed (for example, see Jpn. Pat. Appln. KOKAI Publication No. 8-65211). However, with this method, high-frequency echo components remain unremoved.